Such sensor devices are used especially in motor vehicles within the framework of driver assistance systems known as park distance control (PDC), in order to assist the driver during maneuvering or parking of the motor vehicle. These PDC systems operate with the aid of ultrasonic distance-measuring devices, which are normally mounted only in the rear section or in the rear section as well as the front section of the vehicle. In particular, the distance with respect to the objects in close proximity is measured, especially the remaining space behind the vehicle, and an optic and/or acoustic signal unit emits a signal as a function of the measured distance for this purpose. The distance may be indicated by, for example, light diodes of different colors and/or by an interval sound, a shortening of the intervals indicating a decrease in the available clearance.
A sensor device of the type mentioned in the introduction is described in German Patent Application No. DE 44 10 895. In particular, it provides a method and a device for the concealed installation of a sensor in an external vehicle component, especially in a safety bumper. For this purpose, a cover is placed on top, which is adapted to the outer contour of the external vehicle component in a form fit and which completely covers the outer vehicle component or, at the very least, covers it at the location where the sensor is installed, so that the vehicle design is not marred by the concealed installation of the sensor. In particular, the material of the cover is discussed, which also may be formed by an appropriately strong foil. According to German Patent Application No. DE 44 10 895, for instance, sufficient hardness or rigidity must be ensured for the cover so that signals from the ultrasonic sensor are damped as little as possible and the sensor is protected from external mechanical effects in a reliable manner.
In addition, German Patent No. DE 296 14 691 describes an ultrasonic sensor whose housing terminates in the form of a sensor cup, which is open at the front end and has a radially inwardly projecting annular edge. Resting against the annular edge, on the inside of the radiation side, is a Teflon foil, which is pressed against the annular edge by an inner sleeve via an inner screw screwed into the housing. The Teflon foil covers only the oscillatory sensor element so as to permit its use also in an aggressive atmosphere. The housing of the sensor is not covered.
Especially in the case of vehicles of the luxury category, there is often a preference that specific external components of the vehicle be chrome-colored, for instance in that the utilized material has a high chrome content, or in that the part is completely made of chrome or coated by a chrome layer. Safety bumpers or fenders are among these chrome-colored external vehicle components. Since ultrasonic sensors for parking aids are mostly considered standard equipment among the luxury class these days, there is the wish to install them also in external vehicle components that are chrome-colored. Here, too, the ultrasonic sensor should be barely noticeable, primarily for aesthetic reasons. For concealed or the most inconspicuous installation in the chrome-colored external vehicle components, the sensors are therefore coated by a primer and then chromed, and thereby receive their chrome color (galvanic coating).
However, this procedure has the serious disadvantage of having a highly adverse effect on the mechanical properties of the outer oscillatory diaphragm of the ultrasonic sensor inasmuch as it results in a changed E module and changed damping.
There is the particular problem that the layer thickness of the primer coat and chrome layer may vary, which causes the signal characteristic of the ultrasonic sensor to vary to different and undesired extents as well. Even if the variation of the layer thickness is able to be kept relatively minimal, it nevertheless leads to relatively large and, above all, unpredictable deviations due to the very narrow frequency band in which the sensor element is operating.
Furthermore, the applied chrome layer changes the mechanical properties of the sensor structure by increasing its rigidity and thus also the natural oscillation frequency. Although the applied mass of the primer coat and the chrome layer leads to a reduction in the natural oscillation frequency, the application of the relatively hard chrome material overcompensates for this effect, so that the natural oscillation frequency of the housing in the form of the sensor cup or diaphragm cup is increased overall, which also increases the transmission frequency. This detunes the sensor. This raising of the transmission frequency must be compensated by additional processing of the diaphragm cup in another machining step, for instance by reducing the layer thickness of the sensor element, especially its diaphragm, by machining already prior to the coating.
It is an objective of the present invention to provide a sensor device which is not only visually adapted to chrome-colored external vehicle components, but also reduces the required processing in connection with such an adaptation. Above all, however, the modifications of the mechanical properties of the sensor device that go hand-in-hand with the optical adaptation to chrome-colored external vehicle components must be reduced or at least kept within predictable limits that are narrower than in the related art. At the same time, it is the objective of the present invention to provide an installation method that allows an especially uncomplicated installation of the sensor device according to the present invention in the external vehicle component.